Conventional infrared emission sources typically employ a wire, metal or ceramic element which is heated to emit a continuous broad band of infrared radiation. However, such infrared sources exhibit a number of difficulties, particularly when they are employed for monitoring the presence of trace molecular species in a target gas sample. In such monitoring systems the concentration of the trace species being monitored is typically determined as a function of the amount of infrared radiation absorbed by the species in characteristic wavelength bands. Because the molecular species present in the sample absorbs radiation in these bands, the resultant reduction in the intensity of the infrared output signal in the bands is measured and used to determine the presence or concentration of the species in the sample.
The sensitivity achieved by systems employing conventional infrared emission sources is sometimes rather poor. This occurs for molecular species where the average spacing of the absorption lines is significantly larger than the average width of the lines. In these instances only a small fraction of the total broad band radiation will be absorbed within the molecular absorption lines. This problem is particularly troublesome at low concentrations of the trace species. Even though infrared filters are used to provide the characteristic absorbed wavelength bands to the sample, these introduced bands are still fairly broad relative to the absorbing linewidths within these bands of the trace species at low concentrations. As a result, the output signal from the sample is comparable in strength to the introduced signal. To determine the trace species concentration the introduced and output signals must be compared and because at low concentrations both are relatively strong signals, such comparison is difficult. Detection has been facilitated somewhat by increasing absorption to reduce the intensity of the sample output signal. However, this has typically required an increased sample path length achieved by employing relatively large, complex and cumbersome multiple reflection optical cells.
Selectivity is also a significant problem because the introduced wavelength bands are fairly wide. Species other than the test species may also absorb in those wavelength bands and therefore the resultant reduction in the output signal may be due in part to the presence of species other than the particular trace species being monitored. Such systems are often not able to distinguish between various species present in the sample and as a result erroneous measurements may be taken.
In an attempt to overcome the selectivity problem, gas correlation spectroscopy has been employed. Therein a first beam of a continuous band of infrared emission is passed through a gas sample containing a trace species, for example HCl, to be monitored. The output signal from this beam indicates absorption by the trace species as well as other impurities which absorb in the same region. At the same time a second beam of the broad band is passed through a filter containing a known amount of just the trace species being monitored. The output from this filter is a signal whose strength is reduced only by the absorption by the trace species. This signal is then passed through the sample where it is further reduced by both the trace species and the impurities in the sample. The two output signals from the sample are then compared and because the reductions due to impurities cancel out, the difference in the signals is due entirely to the presence of the trace species in the sample.
Although gas correlation spectroscopy does improve selectivity somewhat it still requires that two fairly strong signals be compared. Therefore, sensitivity at low concentrations continues to be a problem. Moreover, these systems are unwieldy and require the manufacture and use of a special molecular species filter for each species being monitored.